The present invention relates to electroplated steel plate which is suitable for use as the outside of the body of an automobile. More particularly, it relates to steel plate which is electroplated with a Zn alloy such as a Zn--Ni or a Zn--Fe alloy.
In cold regions where there is much snowfall, it is common to spread salt and sand on the roads in order to melt ice and prevent skidding. When vehicles pass over such roads, pebbles in the sand often fly into the air and strikes the sides of the vehicles. If the paint on the outside of the vehicle body is damaged by the gravel, the steel plate constituting the vehicle body can easily corrode. Therefore, automobile manufacturers have been working on improving the painted coating of steel plate for vehicle bodies and have been investigating the use of surface-treated steel plate which is electroplated with metallic Zn or a Zn alloy.
Steel plate which has been electroplated with a Zn alloy prior to painting is superior to steel plate which has been electroplated with elemental Zn prior to painting with respect to resistance to corrosion beneath the painted layer. When the painted layer of steel plate which is electroplated with Zn is damaged, the painted layer undergoes a large amount of creep, i.e., swelling of the painting layer due to the occurrence of underfilm corrosion.
In contrast, in the case of steel plate which has been electroplated with a Zn alloy, even if the paint layer atop the plating is damaged, the plating alone provides fairly good corrosion protection. Furthermore, the painted layer on steel plate which has been electroplated with a Zn alloy undergoes much less creep when damaged than in the case of steel plate which is electroplated with Zn.
Despite these advantages of painted steel plate which is electroplated with a Zn alloy, such plate has the problem that when the painted layer is subject to impact at a low temperature, the plating tends to peel off the steel plate, and the plating and painted layer may peel off the steel plate over a large area. Should both the painted layer and the plating peel off, the steel plate is left with no protection against corrosion. This tendency for peeling to occur is therefore a great impediment to the use of steel plate which is electroplated with a Zn alloy for vehicle bodies.